An example of a conventional displacement/light-quantity converter for converting a mechanical vibration into a light signal will be described with reference to FIGS. 4 and 5. In the converter shown in FIG. 4, a planar diaphragm 4 is opposed to a light emitting part 2 and a light receiving part 3 at substantially the same distance d from the parts. Diverging light emitted from the light emitting part 2 at a certain emission angle is reflected off the diaphragm 4, and some of the reflected light reaches the light receiving part 3. FIG. 5 illustrates a relationship between the distance d from the light emitting part 2 and the light receiving part 3 to the diaphragm 4 and the quantity of light impinging on the light receiving part 3 in FIG. 4. As shown in FIG. 5(a), if the distance d from the light emitting part 2 and the light receiving part 3 to the diaphragm 4 is d1, which is less than a predetermined distance, the light emitted from the light emitting part 2 at the certain emission angle and reflected off the diaphragm 4 does not impinge on the light receiving part 3. However, as shown in FIG. 5(b), if the distance d is d2, which is larger than d1, the light emitted from the light emitting part 2 at the certain emission angle and reflected off the diaphragm 4 impinges on an area of the light receiving surface of the light receiving part 3.
Furthermore, as shown in FIG. 5(c), if the distance d is d3, which is larger than d2, the light emitted from the light emitting part 2 at the certain emission angle and reflected off the diaphragm 4 impinges on the entire light receiving surface of the light receiving part 3. Therefore, the quantity of light impinging on the light receiving part 3 increases linearly as the distance d increases from d1 to d3.
Furthermore, as shown in FIG. 5(d), if the distance d is d4, which is still larger than d3, the light emitted from the light emitting part 2 at the certain emission angle and reflected off the diaphragm 4 impinges on the entire light receiving surface of the light receiving part 3. However, if the distance d becomes larger than a predetermined value, the ratio of the quantity of light impinging on the light receiving part 3 to the quantity of light reflected off the diaphragm 4 is lowered. Thus, once the distance d becomes larger than the predetermined value d3, the quantity of light impinging on the light receiving part 3 gradually decreases.
FIG. 6 is a graph showing the relationship, illustrated in aforementioned FIGS. 4 and 5, between the distance d from the light emitting part 2 and the light receiving part 3 to the diaphragm 4 and the quantity of light impinging on the light receiving part 3. As can be seen from the graph in FIG. 6, if the central point of the distance from the light emitting part 2 and the light receiving part 3 to the diaphragm 4 is set to a point in the vicinity of d2, the quantity of light varies largely in response to a variation of the distanced. Thus, when the diaphragm 4 is displaced vertically (in the vertical direction of the diaphragm 4 in FIGS. 4 and 5) in a small amount, the quantity of light impinging on the light receiving part 3 changes linearly, and detecting this change can provide detection of such a small displacement.
The conventional displacement/light-quantity converter described above has a problem described below, because the diverging light emitted from the light emitting part 2 at the certain emission angle remains diverging even after being reflected of f the diaphragm 4 when it reaches the light receiving surface of the light receiving part 3. That is, since the diverging light reflected off the diaphragm 4 and impinging on the light receiving surface has diverged radially from the light emitting part 2, the quantity of light impinging on the light receiving surface of the light receiving part 3 is significantly reduced compared to the total quantity of light emitted from the light emitting part 2. Thus, even if the distance from the light emitting part and the light receiving part to the diaphragm is set at a value that allows the quantity of light impinging on the light receiving part 3 to change linearly, it is difficult to significantly enhance sensitivity of the diaphragm 4 to a small displacement. In order to enhance the sensitivity of the diaphragm 4 to a small displacement, the light emitting part 2 and the light receiving part 3 may be brought closer to each other. However, this approach leads to a problem that the flexibility in device design is compromised.
As another approach to enhance the sensitivity of the diaphragm 4 to a small displacement, the diaphragm 4 may be brought closer to the light emitting part 2 and the light receiving part 3. Again, however, this approach compromises the flexibility in device design. In addition, since the diaphragm 4 or the light emitting part 2 and light receiving part 3 need to be moved relative to each other, it is difficult to attain a high adjustment precision.